1. Field of the Invention
The present invention relates to a packaging method for sealing a semiconductor chip and to a packaged chip prepared by such method.
2. Description of the Background Art
Conventionally, thermosetting resins, such as epoxy resins, silicone resins, urethane resins, or the like, are used as sealing materials for semiconductor chips used in electronic parts. They are thus useful resins for effecting such operations as potting, sealing, coating, transfer molding methods, or the like.
However, the difficulty with using conventional sealing materials is that thermosetting resins require a preliminary precuring which lengthens the curing time and requires a two step process involving postcuring. In an attempt to solve this problem, the prior art has suggested using thermoplastic resins, instead of thermosetting resins, and direct injection molding of the thermoplastic into a mold containing the semiconductor.
Reference is made to the methods disclosed, for example, in Japanese Laid Open Patent Application Nos. 248546/1989, 75662/1990, 31451/1988, 106963/1991, U.S. Pat. No. 5,110,861, which all refer to the use of thermoplastic resins instead of thermosetting resins. However, the thermoplastic resins proposed by this prior art have proven to be insufficient. For instance, the use of polyphenylene sulfide, liquid crystal polymers, or similar thermoplastic resins, have the drawback that they are not particularly adherent to the lead frames frequently used in electronic devices, so that fillers are needed to improve adhesiveness. Unfortunately, however, while adhesiveness is improved by the use of fillers, flowability is impaired. To avoid this problem, smaller molecular weight resins must be used, but this, in turn, lowers the strength of the resin. In addition, fillers render the resins opaque and thus interfere with its usefulness as a sealant, which requires good light transmissivity.
In Japanese Patent Laid-open (Kokai) No. 31451/1988 there is disclosed a light-emitting diode which uses an olefinic polymer having an alicyclic structure as a thermoplastic resin. But here again, adhesiveness to the lead frame is a particular problem. Moreover, the use of such resins have the additional problem of possessing inadequate resin heat resistance which cannot withstand the temperatures of soldering, so that the resin often fails during processing of the semiconductor into the electronic device.
Japanese Patent Laid-open (Kokai) No. 105610/1987 discloses another alternative method of sealing with a resin by the RIM formation using a resin formed from a norbornene monomer. The problem with such resins, again, is their inherent poor heat resistance.
Japanese Patent Laid-open (Kokai) No. 106963/1991 discloses still another attempt at solving the problem. In that reference an optical disk substrate is used as a hub into which a metal part is inserted. The insertion is accomplished using a molding resin composition comprising a hydrogenated norbornene ring-opened polymer and rubber. Again this type of resin fails to solve the problem noted above, since blends of resins and rubber are generally opaque and interfere with light transmissivity.
Both Japanese Patent Laid-open (Kokai) No. 72535/1989 and Japanese Patent Laid-open (Kokai) No. 30238/1981 disclose sealing of electronic parts using injection molded poly-4-methyl-1-pentene. However, polymers of 4-methyl-1pentene are characterized by poor adhesiveness to lead. Copolymers of 4-methyl-1-pentene with grafted unsaturated carboxylic acid will improve adhesion, but requires an additional polymerization step for the graft modification, thereby undesirably complicating the production process. Moreover, a consistently high quality product is difficult to achieve and requires simultaneously good purification and efficient grafting, thereby further complicating resin preparation.
The attempt to convert from a thermosetting resin to a thermoplastic resin for injection molding to reduce molding time has therefore not been successful. Either the alternative resin is characterized by insufficient heat resistance, or other poor heat characteristics, such as poor heat cyclability, or it is characterized by poor light transmission properties or poor long term reliability, or the like. Alternatively, such alternative resins have been unsuccessful since they are not easily molded without deformation of the connecting wires or without breakage of the semiconductor chips.